Aspergillus fumigatus is an important opportunistic fungal pathogen that has become the leading infectious mould of immunocompromised patients. Despite some advances in therapy, invasive aspergillosis continues to have a poor outcome, emphasizing the need for more information on fungal pathways that are essential to the growth of the organism in vivo. A. fumigatus secretes tremendous quantities of extracellular proteins as part of its normal saprophytic lifestyle, which places it at high risk for protein unfolding and endoplasmic reticulum (ER) stress. The unfolded protein response (UPR) counters this stress by upregulating the expression of genes involved in protein folding, secretion and degradation. In this grant, we provide preliminary data that A. fumigatus relies heavily upon the UPR to support the secretory activity that it needs to survive ER stress and acquire nutrients from host tissue. Secondly, we demonstrate that loss of UPR function exhibits potent synergism with existing antifungal drugs against A. fumigatus. Using mutants of the UPR and the related ER- associated degradation pathway (ERAD), we propose to validate ER stress responses as a therapeutic target for aspergillosis, and to gain insight into the key components of the response. The following specific Aims are proposed: Aim I will test the hypothesis that the UPR is essential for the growth of A. fumigatus under conditions of ER stress, and that it contributes to the ability of the organism to cause disease in vivo. Aim II will test the hypothesis that ERAD is also required to sustain growth under conditions of ER stress, and that it works in concert with the UPR to support the virulence of the organism in vivo. Aim III will use a genome-wide approach to identify novel components of the A. fumigatus ER stress response by testing the hypothesis that the response of A. fumigatus to ER stress involves specific changes in the transcription and translation of a subset of mRNAs. These studies have the potential to identify novel targets for the treatment of aspergillosis, and have broader implications for increasing the antifungal susceptibility profile of fungal species that are intrinsically resistant to existing antifungals.